Mitochondria in vitro

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Evolutionary Origin of Mitochondria

Unlike any other organelle, except
for chloroplasts, mitochondria appear to originate
only from other mitochondria. They contain their
own DNA, which is circular as is true with bacteria,
along with their own transcriptional and translational
machinery. Mitochondrial ribosomes and transfer
RNA molecules are similar to those of bacteria,
as are components of their membrane.These and related
observations led Dr. Lynn Margulis, in the 1970s,
to propose an extracellular origin for mitochondria.

Some species of present day protists
contain living organisms within their cytoplasm.
For example, Paramecium bursaria are hosts
for zoochlorellae, photosynthetic protists, that
reside within the cytoplasm. The relationship appears
to be symbiotic. The endosymbiont gains protection
and possibly some essential nutrients from the
host cytoplasm. The host has a readily available
food source when its usual food source is depleted.
If you have the opportunity to observe P. bursaria,
note that the endosymbionts are not incorporated
into food vacuoles. They are residents within the
cytoplasm itself, and either are descended from
organisms that survived endocytosis or have some
mechanism for escaping food vacuoles once they
are ingested.

Protists are eukaryotes, of course, meaning
that their genetic material is organized into
a compartment, the nucleus, that is surrounded
by membrane, and that they have membrane-delineated
organelles. In the warm seas of the ancient earth,
the first living things would have been prokaryotes.
The endosymbiotic hypothesis for the origin of
mitochondria (and chloroplasts) suggests that
mitochondria are descended from specialized bacteria
(probably purple nonsulfur bacteria) that somehow
survived endocytosis by another species of prokaryote
or some other cell type, and became incorporated
into the cytoplasm. The ability of symbiont bacteria
to conduct cellular respiration in host cells
that relied on glycosis and fermentation would
have provided a considerable evolutionary advantage.
Similarly, host cells with symbiont bacteria
capable of photosynthesis would also have an
advantage. In both cases, the number of environments
in which the cells could survive would have been
greatly expanded.

Mitochondria do not contain anywhere near the amount
of DNA needed to code for all mitochondria-specific
proteins, however, a billion or so years of evolution
could account for a progressive loss of independence.
The endosymbiotic hypothesis might be called a
theory, but experimental evidence can't be provided
to test it. Only circumstantial evidence is available
in support of the proposal, which is the most likely
explanation for the origin of mitochondria. The
evidence needed to change the model from hypothesis
to theory is likely forever lost in antiquity.

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Created by David R. Caprette (caprette@rice.edu), Rice University 19
Jun 00Updated 26 May 05